The present invention relates to a process and to an apparatus for purifying waste waters.
In some cases, where the impurities in industrial waste waters cannot be biologically degraded, there only remains combustion to total oxidation of the impurities. Such combustion requires great expenditures in energy for the evaporation of the amount of water involved. Furthermore, the subsequent purification of the large volumes of gas thus produced involves high equipment costs. Processes already have been proposed wherein multistage evaporation of the water is utilized which concentrates the impurities into a residue which then can at least, in part, be burned. The heat released, for instance, from organic impurities during such a combustion, forms the main source of energy for the concentration process. However, when the water component is being evaporated, the more volatile impurities also pass into the vapor phase, whereby the water of this vapor phase, obtained from a multistage evaporation system, again includes impurities to such an extent as to require further purification, utilizing, for example, activated carbon or other expensive procedures.
This drawback is eliminated by the process of the present invention is a particularly advantageous and economical manner wherein the proportion of low-boiling point impurities in the waste water is adjusted to a predetermined value, whereupon part of the impurities of the waste water following an evaporation process is concentrated into a still fluid residue while the gaseous impurities leaving the evaporation stage together with the waste vapors are brought to spontaneous combustion and hence to complete oxidation by treating them with heat and air. Appropriately, the waste vapors strongly heated during this oxidation are used to transfer said heat to the waste vapors introduced into oxidation. A particularly economical operation can be achieved in that the waste vapors expelled from the oxidation stage and consisting of pure water vapor and inertly behaving gases, following preheating of the waste vapors arriving at the oxidation stage, are cooled approximately to the condensation temperature in a subsequent step, for instance by the spray injection of water, whereupon they release their heat content by heat-exchange condensation, whether for the purpose of operating further evaporation stages or the generation of useful steam. The residue concentrate obtained from one or more evaporation stages may also be burned, for instance, to obtain heat for the evaporation procedure with or without the use of auxiliary fuels, or else it may be dried and stored. Again, waste vapors containing impurities of relatively low-boiling points obtained, for instance, from the pretreatment of the waste water or from a first evaporation stage may be fed to combustion for instance by supplying heat to the evaporation process. In every case, the result is that the water vapor departing from the waste-vapor oxidation after its condensation supplies pure water practically free of organic or inorganic impurities.
The apparatus for implementing the process of the present invention is provided with thermal equipment for cleaning the waste water to a predetermined content of low-boiling point impurities, with an evaporation system to concentrate the high-boiling-point impurities and for evaporation the low-boiling impurities, and also with equipment for oxidizing the combustible and oxidizable impurities still contained in the waste vapors leaving the evaporation system and previously heated to spontaneous combustion.